Various techniques are used for effecting separation using asymmetric or composite membranes including selective permeation, ultrafiltration and reverse osmosis. One example of reverse osmosis separation is a desalination process in which seawater, contaminated water, or brackish water is rendered potable. Desalination of such water is often necessary to provide large amounts of relatively nonsalty water for industrial, agricultural, or home use. Such desalination can be effected by forcing water through a reverse osmosis membrane which retains the contaminants or salts. Typical reverse osmosis membranes have a very high salt rejection coefficient and possess the ability to pass a relatively large amount of water through the membrane at relatively low pressures.
Various U.S. patents describe reverse osmosis membranes and supports that are useful in industrial processes. See for example: U.S. Pat. Nos. 4,830,885, 3,744,642, 4,277,344, and 4,619,767.
U.S. Pat. Nos. 4,765,897 and 4,812,270 discusses polyamide membranes that are reported to be useful for water softening applications. The membranes are prepared by treating a polyamide reverse osmosis membrane with a strong mineral acid followed by treatment with a rejection enhancing agent.
Subsequently, U.S. Pat. No. 4,983,291 reported semipermeable membranes that are subjected to treatment with acids or certain acid derivatives and dried to provide a membrane that maintains high flux with a concurrent rejection rate.
Despite the above disclosures, there remains a need for reverse osmosis membranes having useful flux and retention properties. In particular, there is a need for reverse osmosis membranes that possess improved flux characteristics while maintaining useful rejection characteristics.